This invention relates to a silver halide photographic material and, more particularly, it is concerned with a silver halide photographic material sensitized by chemical ripening of a silver halide emulsion containing silver halide grains mainly composed of silver iodobromide.
As photographic silver halide, there have been previously put to practical use a wide variety of silver halides such as silver chloride, silver bromide, silver chlorobromide, silver iodobromide, silver chloroiodobromide and the like; particularly, silver iodobromide has been employed as a silver halide for a highly sensitive photograph to prepare a highly sensitive emulsion.
Recently, a demand has become more severe for a silver halide emulsion for a highly sensitive photograph and a increasingly higher level of demand has been placed for photographic performances such as higher sensitivity, superior graininess, higher sharpness, lower fogging density, sufficiently higher optical density and the like. On the other hand, a strong demand for development of a lower silver content photosensitive material has occured in view of indication of a recent exhaustion of silver resourse. These seemingly different demands could be probably met by technique for preparing a silver halide emulsion with low fogging and high sensitivity. Therefore, it may be said that development of a silver iodobromide-type silver halide emulsion with lower fogging and higher sensitivity is the most important subject in this field.
In order to increase sensitivity of a silver halide photographic emulsion, there have been proposed various chemical sensitization methods wherein chemical ripening is carried out in the presence of a variety of chemical substances; typically, there is known chemical ripening using sulfur sensitization, selenium sensitization, noble metal sensitization, reduction sensitization or any combination thereof.
Of the above sensitization methods, selenium sensitization is disclosed, for example, in U.S. Pat. Nos. 1,574,944, 1,623,499, 1,602,592, 2,642,361, 2,739,060, 3,297,446, 3,420,670, 3,320,069, 3,658,540, 3,408,196, 3,408,197, 3,442,653, 3,591,385; British Pat. Nos. 255,846, 861,984; West German Pat. Nos. 10 33 510, 15 47 762; French Pat. Nos. 2,093,038, 2,093,209; Japanese Patent Publication Nos. 34491/1977, 34492/1977, 295/1978, 36009/1977, 38409/1977, 22090/1982. However, selenium sensitization could generally provide a greater sensitization effect as compared with sulfur sensitization commonly practised in the art at the present time, but the former sensitization has drawbacks in easily producing fogging and also soft gradation; hence it is said to be difficult in practical use. On the other hand, a method wherein silver iodide incorporated into silver bromide or silver chlorobromide up to such a range to form a solid solution or some mixed crystals, has been proposed for increasing sensitivity inherent in silver halide grain itself. For instance, A. P. H. Tripeli and W. F. Smith reported in Photographic Journal, 79, 463 (1939) that, where silver iodide is incorporated into silver bromide, a sensitivity could be increased as its silver iodide content is increased up to a certain content.
However, silver halide grains having silver iodide incorporated therein have a drawback of showing soft gradation as its silver iodide content is increased.
As explained above, both selenium sensitization and application of silver iodide for increase in sensitivity, which could be expected as promising measures for higher sensitization, could not adequately control other important photographic property, i.e., gamma (.gamma.) to provide soft gradation. This has led to closure against utilization of both measures, thus disturbing the development of a photographic material.